The photographic process utilizing a silver halide shows excellent photographic properties such as sensitivity and gradation adjustability as compared to other photographic processes such as electrophotography and diazo photographic processes and, thus, has long been put into wide spread commercial application. In recent years, in processes for the formation of images on a light-sensitive material using a silver halide, techniques for simple and rapid formation of images have been developed by switching from the conventional wet processes using a developing agent to dry processes developed by heating or similar means.
Heat developable light-sensitive materials are well-known in the art. Examples of heat developable light-sensitive materials and their concomitant processes are described in, for example, SHASHIN KOGAKU NO KISO (published by Corona, pp. 553-555), Nebletts' Handbook of Photography and Reprography, (7th edition, published by Van Northrand Reinhold Company, pp. 32-33), U.S. Pat. Nos. 3,152,194, 3,301,678, 3,392,020, and 3,457,075, British Patents Nos. 1,131,108 and 1,167,777 and Research Disclosure RD No. 17029 (June-1978 issue, pp. 9-15).
Many processes for obtaining color images have been proposed. Processes for forming color images by a coupling reaction of an oxidation product of a developing agent and a coupler have been proposed. For example, the coupling of a p-phenylenediamine reducing agent and a phenol-based or active methylene coupler is proposed in U.S. Pat. No. 3,531,286. Further, a p-aminophenol-based reducing agent is proposed in U.S. Pat. No. 3,761,270, a sulfonamidophenol-based reducing agent is proposed in Belgian Patent No. 802,519 and Research Disclosure (September-1975 issue, pp. 31-32), and a combination of a sulfonamidophenol-based reducing agent and a 4-equivalent coupler is proposed in U.S. Pat. No. 4,021,240.
As to processes for the formation of positive color images by a light-sensitive silver dye bleaching process, examples of useful dyes and bleaching processes are described in Research Disclosure, No. 14433 (April-1976 issue, pp. 30-32) and Research Disclosure, No. 15229 (December-1976 issue, pp. 14-15) and U.S. Pat. No. 4,235,957.
Furthermore, examples of heat developing image formation processes utilizing a compound which originally contains a dye portion and can release a mobile dye in correspondence to or in counter-correspondence to a reduction reaction of a silver halide to silver at an elevated temperature, are disclosed in, for example, European Patent Nos. 76,492A and 79,056A and Japanese patent application (OPI) Nos. 28928/83 and 26008/83 (the term "OPI" as used herein refers to a "published unexamimed application").
A silver halide used in such a heat developable light-sensitive material is spectrally sensitized as necessary, as described in Research Disclousre, No. 17029 (June-1978 issue). However, unlike the conventional light-sensitive material which is developed by a processing solution containing a large amount of water, the heat developable light-sensitive material cannot remove "used" or previously reacted sensitizing dye therefrom by dissolving it into a treatment solution. Therefore, such "used" sensitizing dyes remain in the light-sensitive material after the heat development process. In the embodiment described in European Patent No. 76,492A in which a dye is transferred to a dye fixing material to obtain an image, a sensitizing dye is transferred to the dye fixing material together with the dye. This embodiment is disadvantageous in that the presence of a sensitizing dye may give rise to troublesome stains to images developed.
In order to eliminate such disadvantages, it is proposed in Japanese patent application (OPI) No. 111239/85 to employ a cyanine dye containing an atomic group which renders the dye capable of being faded upon application of heat to the photothermographic material. However, prior art "heat-fading dyes" as described in the above-mentioned patent are disadvantageous in some respects. For example, these "heat-fading" dyes are not fully capable of spectrally sensitizing a silver halide.